…ction in the fragment shader

API changes:
- Added ImGui_ImplVulkan_ReCreateMainPipeline(...) to explicitly re-create the main window pipeline (when some of its properties are changed).
- ImGui_ImplVulkan_CreateMainPipeline(...) does not implicitly use ImGui_ImplVulkan_InitInfo::PipelineRenderingCreateInfo, but a function parameter.
- The main window pipeline is created only if possible during ImGui_ImplVulkan_Init(...) (if a render pass or rendering info are given), else it should be created with ImGui_ImplVulkan_ReCreateMainPipeline(...) before rendering)
Concerning color correction:
- The default behavior of imgui_impl_vulkan does not change (no color correction is applied).
- A color correction method is decided at pipeline compile time (through a vulkan specialization constant)
- Color correction parameters can either be set at pipeline compile time (static mode), or through push_constant (dynamic mode, default)
- A gamma correction mode is implemented (and an extra alpha gamma correction).
- New color modes can easily be added.
Concerning multi view ports:
- Secondary viewports surface format and present mode can now be controlled by the application view ImGui_ImplVulkan_RequestSecondaryViewportsChanges(...)
- Secondary viewports can also use color correction if they use the application requested format.
- Each viewport now has its own VkRenderPass and VkPipeline (cached in the backed data). This fits to the fact that each viewport might have a different format (the previous code was making this (quite reasonable) assumption).
- Recycle some viewport resource on swapchain recreation if possible (CommandPool, CommandBuffer, Fence, Semaphores) rather than recreating them every time.